Automatic transmission and control method of automatic transmission

ABSTRACT

A dual clutch automatic transmission includes a one-way clutch provided in a driving force transmission path that establishes a certain gear ratio in a first set of gear ratios. A driving transmission direction of the one-way clutch is set such that a rotation inputted from a wheel side to an output member in a predetermined rotational direction is transmitted to the input shaft. The predetermined rotational direction corresponds to a backward movement of a vehicle. On condition that the traveling state of the vehicle reaches a predetermined deceleration state, switching control of setting the gears for the first set to a neutral position can be executed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2016-037932, filed on Feb. 29, 2016, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to the automatic transmission of a vehicle.

Description of the Related Art

As an automatic transmission, a so-called dual clutch transmission is known. The dual clutch transmission includes two transmission mechanisms that implement different gear ratios, and connects/disconnects the two transmission mechanisms to/from a driving source such as an internal combustion engine, thereby suppressing interruption of power transmission to the drive wheels at the time of gear change. A transmission that maintains the stop state of a vehicle using interlock of the transmission is also known. Japanese Patent No. 5081215 discloses, in a dual clutch transmission, intentionally putting the vehicle in two speed gears for parking lock, thereby interlocking the transmission to maintain the stop state of a vehicle.

Depending on an arrangement of a transmission, interlock may occur when the vehicle reaches a special traveling state. In such a case, it may be difficult to immediately cancel the interlock due to a jam in an engagement mechanism or the like. Since functions of the vehicle are partially lost upon interlock generation, generation of unintended interlock needs to be prevented.

SUMMARY OF THE INVENTION

It is an object of the present invention to prevent generation of unintended interlock.

According to an aspect of the present invention, there is provided an automatic transmission comprising: a first transmission mechanism to which a driving force of a driving source is input through a first clutch and which is configured to switch driving force transmission paths to an output member to establish a first set of gear ratios; a second transmission mechanism to which the driving force of the driving source is input through a second clutch and which is configured to switch driving force transmission paths to the output member to establish a second set of gear ratios; and a control unit, wherein a one-way clutch is provided in a driving force transmission path that establishes a first certain gear in the first set, the first transmission mechanism comprises: an input shaft to which the driving force of the driving source is input through the first clutch; a first transmission gear provided on the input shaft and configured to establish the first certain gear ratio; a plurality of second transmission gears provided on the input shaft and configured to establish remaining gear ratios in the first set; and a switching mechanism configured to perform engagement and disengagement between the input shaft and the plurality of second transmission gears, a driving transmission direction of the one-way clutch is set such that a rotation inputted from a wheel side to the output member in a predetermined rotational direction is transmitted to the input shaft, the predetermined rotational direction corresponding to a backward movement of a vehicle, and the control unit can execute switching control of causing the switching mechanism to perform the disengagement between the input shaft and the plurality of second transmission gears on condition that a traveling state of the vehicle reaches a predetermined deceleration state.

According to another aspect of the present invention, there is provided a control method of an automatic transmission which comprises: a first transmission mechanism to which a driving force of a driving source is input through a first clutch and which is configured to switch driving force transmission paths to an output member to establish a first set of gear ratios; and a second transmission mechanism to which the driving force of the driving source is input through a second clutch and which is configured to switch driving force transmission paths to the output member to establish a second set of gear ratios, wherein a one-way clutch is provided in the driving force transmission path that establishes a first certain gear ratio in the first set, the first transmission mechanism comprises: an input shaft to which the driving force of the driving source is input through the first clutch; a first transmission gear provided on the input shaft and configured to establish the first certain gear ratio; a plurality of second transmission gears provided on the input shaft and configured to establish remaining gear ratios in the first set; and a switching mechanism configured to perform engagement and disengagement between the input shaft and the plurality of second transmission gears, and a driving transmission direction of the one-way clutch is set such that a rotation inputted from a wheel side to the output member in a predetermined rotational direction is transmitted to the input shaft, the predetermined rotational direction corresponding to a backward movement of a vehicle, the control method comprising: determining whether a traveling state of the vehicle reaches a predetermined deceleration state; and upon determining that the traveling state reaches the predetermined deceleration state, executing switching control of causing the switching mechanism to perform the disengagement between the input shaft and the plurality of second transmission gears.

Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an automatic transmission according to an embodiment of the present invention;

FIG. 2 is a block diagram of the control unit of the automatic transmission shown in FIG. 1;

FIG. 3 is an explanatory view of a torque flow at the time of a stop on a climbing hill;

FIGS. 4A and 4B are explanatory views showing an example of the arrangement of a dog clutch

FIGS. 5A to 5C are flowcharts showing examples of processing of the control unit;

FIGS. 6A and 6B are explanatory views for explaining unintended interlock generation;

FIG. 7 is an explanatory view showing an example of control at the time of abrupt deceleration; and

FIG. 8 is a flowchart showing an example of processing of the control unit.

DESCRIPTION OF THE EMBODIMENTS

<Arrangement of Automatic Transmission>

FIG. 1 is a schematic view (skeleton diagram) of an automatic transmission 1 according to an embodiment of the present invention. The automatic transmission 1 decelerates the rotation driving force output from a driving source 2, outputs the force to a final reduction gear 3, and drives, through left and right drive shafts 4, wheels 5 (only one side is illustrated in FIG. 1) that are left and right drive wheels. The final reduction gear 3 includes, for example, differential gears connected to the drive shafts 4.

The automatic transmission 1 is mounted on, for example, a midship engine/rear drive type vehicle. The number of gear ratios of the automatic transmission 1 can appropriately be designed. In this embodiment, the automatic transmission 1 has nine forward gears and one reverse gear, as will be described later. The driving source 2 is an internal combustion engine here, but may be an electric motor or a unit that combines an internal combustion engine and an electric motor. The driving force of the driving source 2 is input to the automatic transmission 1 through a flywheel 2 a and an output shaft 2 b.

The automatic transmission 1 is a dual clutch transmission including two transmission mechanisms 10 and 20 that implement different gear ratios, and a transmission mechanism 30 that implements the reverse gear. The driving force of the driving source 2 is input to the transmission mechanism 10 through a clutch C1. The transmission mechanism 10 switches driving force transmission paths to an output member 41, thereby establishing gear ratios (the first, third, fifth, seventh, and ninth speed gear ratios in this embodiment) corresponding to the set of odd-numbered speed gear ratios. The driving force of the driving source 2 is input to the transmission mechanism 20 through a clutch C2. The transmission mechanism 20 switches driving force transmission paths to the output member 41, thereby establishing the gear ratios (the second, fourth, sixth, and eighth speed gear ratios in this embodiment) corresponding to the set of even-numbered speed gear ratios.

Each of the clutches C1 and C2 connects/disconnects the transmission of the driving force to/from the corresponding one of the transmission mechanisms 10 and 20. Each of the clutches C1 and C2 is, for example, a wet-type multiple clutch, and includes an outer clutch plate concentrically and integrally attached to the output shaft 2 b, an inner clutch plate connected to a corresponding one of the transmission mechanisms 10 and 20, an actuator that presses the inner clutch plate against the outer clutch plate, a return spring that separates the inner clutch plate from the outer clutch plate, and the like.

The transmission mechanism 10 includes an input shaft 11, a gear G1 and a plurality of gear members 12 to 15 which are provided to be coaxial with respect to the input shaft 11, and a switching mechanism 16. The input shaft 11 is rotatably supported by a transmission case (not shown). The input shaft 11 is connected to the inner clutch plate of the clutch C1, and the driving force of the driving source 2 is input to the input shaft 11 through the clutch C1. The gear G1 is the transmission gear for the first speed gear ratio, which is provided on the input shaft 11. The gear G1 is provided (for example, key-connected or spline-connected) to be unrotatable relative to the input shaft 11, and always rotates during the rotation of the input shaft 11.

The gear members 12 to 15 are cylindrical members provided on the input shaft 11 to be relatively rotatable. A gear G3 that is the transmission gear for the third speed gear ratio is formed on the gear member 12. A gear G5 that is the transmission gear for the fifth speed gear ratio is formed on the gear member 13. A gear G7 that is the transmission gear for the seventh speed gear ratio is formed on the gear member 14. A gear G9 that is the transmission gear for the ninth speed gear ratio is formed on the gear member 15.

The switching mechanism 16 engages/disengages the gear members 12 to 15 (that is, the gears G3, G5, G7, and G9) with/from the input shaft 11. By engagement, an engaged gear member becomes unrotatable relative to the input shaft 11 and always rotates during the rotation of the input shaft 11. By disengagement, a disengaged gear member becomes rotatable relative to the input shaft 11.

In this embodiment, the switching mechanism 16 includes switching units 16 a and 16 b. The switching units 16 a and 16 b can have any structure. In this embodiment, the switching units 16 a and 16 b are each formed from a dog clutch, in particular, a dog clutch with a synchronization mechanism.

The switching unit 16 a engages/disengages the gear members 12 and 13 with/from the input shaft 11. As the operation state of the switching unit 16 a, one of three states, that is, a state (third speed gear ratio in-gear state) in which the gear member 12 engages with the input shaft 11, a state (fifth speed gear ratio in-gear state) in which the gear member 13 engages with the input shaft 11, and a state (neutral) in which both the gear members 12 and 13 disengage from the input shaft 11 can be selected.

The switching unit 16 b engages/disengages the gear members 14 and 15 with/from the input shaft 11. As the operation state of the switching unit 16 b, one of three states, that is, a state (seventh speed gear ratio in-gear state) in which the gear member 14 engages with the input shaft 11, a state (ninth speed gear ratio in-gear state) in which the gear member 15 engages with the input shaft 11, and a state (neutral) in which both the gear members 14 and 15 disengage from the input shaft 11 can be selected.

Additionally, a gear member 27 is coaxially provided on the input shaft 11. The gear member 27 is a cylindrical member provided on the input shaft 11 to be relatively rotatable, and is connected to the inner clutch plate of the clutch C2. A gear 27 a is formed on the gear member 27 and always meshes with a gear 33 of the transmission mechanism 30.

The transmission mechanism 30 includes an input shaft 31, a gear member 32, and a switching mechanism 34. The input shaft 31 is provided in parallel to the input shaft 11 and rotatably supported by the transmission case (not shown). The gear 33 is provided to be unrotatable relative to the input shaft 31. The gear member 32 is a cylindrical member provided on the input shaft 31 to be relatively rotatable. A gear GR that is the transmission gear for the reverse gear is formed on the gear member 32. The gear GR always meshes with the gear G3.

The switching mechanism 34 engages/disengages the gear member 32 (that is, the gear GR) with/from the input shaft 31. By engagement, the gear member 32 becomes unrotatable relative to the input shaft 31 and always rotates during the rotation of the input shaft 31. By disengagement, the gear member 32 becomes rotatable relative to the input shaft 31. In this embodiment, the switching mechanism 34 is also formed from a dog clutch with a synchronization mechanism. As the operation state of the switching mechanism 34, one of two states, that is, a state (reverse gear in-gear state) in which the gear member 32 engages with the input shaft 31 and a state (neutral) in which the gear member 32 disengages from the input shaft 31 can be selected.

The transmission mechanism 20 includes an input shaft 21, a gear 28 and a plurality of gear members 22 to 25 which are provided to be coaxial with respect to the input shaft 21, and a switching mechanism 26. The input shaft 21 is provided in parallel to the input shaft 11 and rotatably supported by the transmission case (not shown).

The gear 28 is provided to be unrotatable relative to the input shaft 21, and always meshes with the gear 33. Hence, the driving force of the driving source 2 is input to the input shaft 21 through the clutch C2 and the gears 33 and 28.

The gear members 22 to 25 are cylindrical members provided on the input shaft 21 to be relatively rotatable. A gear G2 that is the transmission gear for the second speed gear ratio is formed on the gear member 22. A gear G4 that is the transmission gear for the fourth speed gear ratio is formed on the gear member 23. A gear G6 that is the transmission gear for the sixth speed gear ratio is formed on the gear member 24. A gear G8 that is the transmission gear for the eighth speed gear ratio is formed on the gear member 25.

The switching mechanism 26 engages/disengages the gear members 22 to 25 (that is, the gears G2, G4, G6, and G8) with/from the input shaft 21. By engagement, an engaged gear member becomes unrotatable relative to the input shaft 21 and always rotates during the rotation of the input shaft 21. By disengagement, a disengaged gear member becomes rotatable relative to the input shaft 21.

In this embodiment, the switching mechanism 26 includes switching units 26 a and 26 b. Like the switching mechanism 16, the switching units 26 a and 26 b can have any structure. In this embodiment, the switching units 26 a and 26 b are each formed from a dog clutch, in particular, a dog clutch with a synchronization mechanism.

The switching unit 26 a engages/disengages the gear members 22 and 23 with/from the input shaft 21. As the operation state of the switching unit 26 a, one of three states, that is, a state (second speed gear ratio in-gear state) in which the gear member 22 engages with the input shaft 21, a state (fourth speed gear ratio in-gear state) in which the gear member 23 engages with the input shaft 21, and a state (neutral) in which both the gear members 22 and 23 disengage from the input shaft 21 can be selected.

The switching unit 26 b engages/disengages the gear members 24 and 25 with/from the input shaft 21. As the operation state of the switching unit 26 b, one of three states, that is, a state (sixth speed gear ratio in-gear state) in which the gear member 24 engages with the input shaft 21, a state (eighth speed gear ratio in-gear state) in which the gear member 25 engages with the input shaft 21, and a state (neutral) in which both the gear members 24 and 25 disengage from the input shaft 21 can be selected.

In this embodiment, the output member 41 is a gear provided on a countershaft 40 to be relatively unrotatable. The countershaft 40 is provided in parallel to the input shaft 11 and rotatably supported by the transmission case (not shown). In addition to the output member 41, gears 42 to 46 are coaxially provided on the countershaft 40. The gears 42 to 45 are provided to be unrotatable relative to the countershaft 40. The output member 41 always meshes with the gear G3. The gear 42 always meshes with the gear G2. The gear 43 always meshes with the gears G4 and G5. The gear 44 always meshes with the gears G6 and G7. The gear 45 always meshes with the gears G8 and G9.

The gear 46 is provided on the countershaft 40 through a one-way clutch OC. The one-way clutch OC rotates in one direction for driving transmission. The gear 46 always meshes with the gear G1. That is, the one-way clutch OC is provided in the driving force transmission path that establishes the first speed gear ratio.

The one-way clutch OC is a sprag one-way clutch as an example in this embodiment, and the driving transmission direction is set as follows. The rotation direction of the countershaft 40 during forward traveling of the vehicle will be referred to as a forward rotation direction here.

In this embodiment, if a rotation speed V1 of the gear 46 in the forward rotation direction exceeds a rotation speed V2 of the countershaft 40 in the forward rotation direction, a pivotal movement in the engaging direction is given to a sprag OC3 between an outer race OC1 and an inner race OC2 to set the one-way clutch OC in an engaging state, and the driving force of the input shaft 11 is transmitted to the countershaft 40. Hence, if the switching units 16 a and 16 b are at the neutral positions, the driving force of the input shaft 11 is transmitted to the countershaft 40 through the gears G1 and 46 and the one-way clutch OC during acceleration.

On the other hand, if the rotation speed V1 of the gear 46 in the forward rotation direction is less than the rotation speed V2 of the countershaft 40 in the forward rotation direction, the pivotal movement in the engaging direction is not given to the sprag OC3, and the outer race OC1 and the inner race OC2 are set in a non-engaging state. Hence, if one of the switching units 16 a and 16 b is set in the engaging state, the driving force of the input shaft 11 is transmitted to the countershaft 40 not through the gears G1 and 46 and the one-way clutch OC but through another path. For example, in the third speed gear ratio in-gear state, the driving force of the input shaft 11 is transmitted to the countershaft 40 through the gears G3 and 41. This also applies to the fifth speed gear ratio, the seventh speed gear ratio, and the ninth speed gear ratio.

With the arrangement that establishes the first speed gear ratio by the one-way clutch OC, the switching mechanism for the first speed gear ratio can be omitted.

The output member 41 always meshes with a gear 51. The gear 51 is provided on an output shaft 50 to be relatively unrotatable. The output shaft 50 is provided in parallel to the input shaft 11 and rotatably supported by the transmission case (not shown). A bevel gear 52 is also provided on the output shaft 50 to be relatively unrotatable. The bevel gear 52 always meshes with a bevel gear 3 a of the final reduction gear 3. The driving force of the output shaft 50 is transmitted to the drive shafts 4 and the wheels 5 through the final reduction gear 3.

Note that the automatic transmission 1 can be provided with a parking lock device. The parking lock device may lock, for example, the countershaft 40 or the output shaft 50 on the transmission case.

The driving force transmission path in each gear ratio will be described next.

When the first speed gear ratio is selected, all the switching mechanisms 16, 26, and 34 are set at the neutral positions, the clutch C1 is set in the connected state, and the clutch C2 is set in the released state. The driving force of the driving source 2 is transmitted through the path of clutch C1→input shaft 11→gear G1→gear 46→one-way clutch OC→countershaft 40→output member 41→gear 51→output shaft 50.

When the second speed gear ratio is selected, all the switching mechanisms 16 and 34 and the switching unit 26 b are set at the neutral positions, and the switching unit 26 a sets the second speed gear ratio in-gear state. The clutch C2 is set in the connected state, and the clutch C1 is set in the released state. The driving force of the driving source 2 is transmitted through the path of clutch C2→gear member 27 (gear 27 a)→gear 33→gear 28→input shaft 21→gear G2→gear 42→countershaft 40→output member 41→gear 51→output shaft 50.

When the third speed gear ratio is selected, all the switching mechanisms 26 and 34 and the switching unit 16 b are set at the neutral positions, and the switching unit 16 a sets the third speed gear ratio in-gear state. The clutch C1 is set in the connected state, and the clutch C2 is set in the released state. The driving force of the driving source 2 is transmitted through the path of clutch C1→input shaft 11→gear G3→output member 41→gear 51→output shaft 50.

When the fourth speed gear ratio is selected, all the switching mechanisms 16 and 34 and the switching unit 26 b are set at the neutral positions, and the switching unit 26 a sets the fourth speed gear ratio in-gear state. The clutch C2 is set in the connected state, and the clutch C1 is set in the released state. The driving force of the driving source 2 is transmitted through the path of clutch C2→gear member 27 (gear 27 a)→gear 33→gear 28→input shaft 21→gear G4→gear 43→countershaft 40→output member 41→gear 51→output shaft 50.

When the fifth speed gear ratio is selected, all the switching mechanisms 26 and 34 and the switching unit 16 b are set at the neutral positions, and the switching unit 16 a sets the fifth speed gear ratio in-gear state. The clutch C1 is set in the connected state, and the clutch C2 is set in the released state. The driving force of the driving source 2 is transmitted through the path of clutch C1→input shaft 11→gear G5→gear 43→countershaft 40→output member 41→gear 51→output shaft 50.

When the sixth speed gear ratio is selected, all the switching mechanisms 16 and 34 and the switching unit 26 a are set at the neutral positions, and the switching unit 26 b sets the sixth speed gear ratio in-gear state. The clutch C2 is set in the connected state, and the clutch C1 is set in the released state. The driving force of the driving source 2 is transmitted through the path of clutch C2→gear member 27 (gear 27 a)→gear 33→gear 28→input shaft 21→gear G6→gear 44→countershaft 40→output member 41→gear 51→output shaft 50.

When the seventh speed gear ratio is selected, all the switching mechanisms 26 and 34 and the switching unit 16 a are set at the neutral positions, and the switching unit 16 b sets the seventh speed gear ratio in-gear state. The clutch C1 is set in the connected state, and the clutch C2 is set in the released state. The driving force of the driving source 2 is transmitted through the path of clutch C1→input shaft 11→gear G7→gear 44→countershaft 40→output member 41→gear 51→output shaft 50.

When the eighth speed gear ratio is selected, all the switching mechanisms 16 and 34 and the switching unit 26 a are set at the neutral positions, and the switching unit 26 b sets the eighth speed gear ratio in-gear state. The clutch C2 is set in the connected state, and the clutch C1 is set in the released state. The driving force of the driving source 2 is transmitted through the path of clutch C2→gear member 27 (gear 27 a)→gear 33→gear 28→input shaft 21→gear G8→gear 45→countershaft 40→output member 41→gear 51→output shaft 50.

When the ninth speed gear ratio is selected, all the switching mechanisms 26 and 34 and the switching unit 16 a are set at the neutral positions, and the switching unit 16 b sets the ninth speed gear ratio in-gear state. The clutch C1 is set in the connected state, and the clutch C2 is set in the released state. The driving force of the driving source 2 is transmitted through the path of clutch C1→input shaft 11→gear G9→gear 45→countershaft 40→output member 41→gear 51→output shaft 50.

When the reverse gear is selected, both the switching mechanisms 16 and 26 are set at the neutral positions, and the switching mechanism 34 sets the in-gear state. The clutch C2 is set in the connected state, and the clutch C1 is set in the released state. The driving force of the driving source 2 is transmitted through the path of clutch C2→gear member 27 (gear 27 a)→gear 33→input shaft 31→gear GR→gear G3→output member 41→gear 51→output shaft 50.

A control unit 60 of the automatic transmission 1 will be described next with reference to FIG. 2. FIG. 2 is a block diagram showing the circuit arrangement of the control unit 60. The control unit 60 is an ECU configured to control the automatic transmission 1, and includes a processing unit 61 such as a CPU, a storage unit 62 such as a RAM and a ROM, and an interface unit 63 that interfaces an external device and the processing unit 61.

The processing unit 61 executes a program stored in the storage unit 62, and controls various kinds of actuators 80 based on the detection results of various kinds of sensors 70. Concerning control examples to be described later, the various kinds of sensors 70 include, for example, an accelerator position sensor 71, a vehicle speed sensor 72, a slope sensor 73, an SP (Shift Position) sensor 74, a brake sensor 75, and a wheel speed sensor 76.

The accelerator position sensor 71 is a sensor that detects, for example, the depression amount of the accelerator pedal by the driver. The vehicle speed sensor 72 is a sensor that detects, for example, the rotation speed of the countershaft 40. The slope sensor 73 detects the slope of the traveling road of the vehicle. The slope sensor 73 can be a sensor configured to directly detect the slope of the traveling road, or an acceleration sensor or a speed sensor. If the slope sensor 73 is an acceleration sensor or a speed sensor, the slope of the traveling road can be calculated based on, for example, the relationship between the accelerator opening and the acceleration or speed of the vehicle.

The SP (Shift Position) sensor 74 is a sensor configured to detect the shift position selected by the driver. As the shift positions, for example, four types of shift positions, that is, the P range (parking range), D range (drive range), N range (neutral range), and R range (reverse range) exist. The D range may include an automatic gear change mode and a manual gear change mode. The brake sensor 75 is a sensor configured to detect a driver's operation on the foot brake. The wheel speed sensor 76 is a sensor configured to detect the rotational speed of the wheels 5.

As another sensor, for example, a sensor configured to detect the in-gear state or off-gear state in each of the switching mechanisms 16, 26, and 34 is usable.

The various kinds of actuators 80 include an actuator that drives the clutches C1 and C2 and actuators provided in the switching units 16 a, 16 b, 26 a, and 26 b and the switching mechanism 34. These actuators are, for example, motors or control valves. In this embodiment, the switching units 16 a, 16 b, 26 a, and 26 b are assumed to be electric actuators.

When the D range is selected, the processing unit 61 selects a gear ratio based on the travelling state of the vehicle in accordance with, for example, a gear change map stored in the storage unit 62. The gear ratio is normally switched one by one. For example, at the time of acceleration, the gear ratio is switched from first speed gear ratio→second speed gear ratio→third speed gear ratio . . . . At the time of deceleration as well, the gear ratio is switched from seventh speed gear ratio→sixth speed gear ratio→fifth speed gear ratio . . . . When switching from an odd-numbered speed gear ratio to an even-numbered speed gear ratio, the clutch C2 is released in advance, and the in-gear state of the next even-numbered speed gear ratio is set. Then, when switching from the odd-numbered speed gear ratio to the even-numbered speed gear ratio, since the next even-numbered speed gear ratio is established by the release of the clutch C1 and the connection of the clutch C2, the gear change time can be shortened. This also applies to switching from an even-numbered speed gear ratio to an odd-numbered speed gear ratio.

<Hill Hold>

During a stop in the D range, the processing unit 61 normally sets the switching mechanisms 16, 26, and 34 at the neutral positions, and releases the clutches C1 and C2. If the driver releases the foot brake during a stop on a climbing hill in the D range, the vehicle may move back. The backward movement of the vehicle is prevented by intentionally causing interlock in the automatic transmission 1. FIG. 3 is an explanatory view of the principle and shows a torque flow at the time of the backward movement of the vehicle.

When the vehicle moves back, driving in a direction reverse to that in a forward movement is given by the drive shafts 4 and transmitted to the countershaft 40, as indicated by T1. The rotation direction of the countershaft 40 at this time is the direction reverse to the above-described forward rotation direction.

The driving transmission direction of the one-way clutch OC is the same as described above. In the above setting, if a rotation speed V3 of the countershaft 40 in the reverse direction exceeds a rotation speed V4 of the gear 46 in the reverse direction, a pivotal movement in the engaging direction is given to the sprag OC3, and the outer race OC1 and the inner race OC2 are set in the engaging state. Hence, the driving force of the backward movement is transmitted to the input shaft 11 through the gears 46 and G1. That is, if a rotation in a direction corresponding to the backward movement of the vehicle is input from the side of the wheels 5 to the output member 41, the rotation is transmitted to the input shaft 11.

If one of the odd-numbered speed gear ratios other than the first speed gear ratio is put in gear, a torque circulation occurs between the countershaft 40 and the input shaft 11, and interlock is generated. T3 in FIG. 3 represents a case in which the gears are put in gear for the third speed gear ratio. Since the first speed gear ratio is always put in gear, putting only one gear ratio in gear suffices. Hence, the state necessary for preventing the backward movement at the time of a stop can be established in a shorter time. The gear ratio to be put in gear may be the fifth speed gear ratio, the seventh speed gear ratio, or the ninth speed gear ratio. When the third speed gear ratio whose ratio is close to the first speed gear ratio is put in gear, the load applied to each shaft can be reduced in some cases.

Note that in this embodiment, the one-way clutch OC is provided between the gear 46 and the input shaft 11, but may be provided between the input shaft 11 and the gear G1 depending on an arrangement of an transmission. In addition, the one-way clutch OC is provided in the driving force transmission path to establish the first speed gear ratio, but may be provided in a driving force transmission path to establish another gear ratio depending on an arrangement of an transmission. For example, the one-way clutch OC may be provided in the driving force transmission path to establish the second speed gear ratio. If the one-way clutch OC is provided in a driving force transmission path to establish an even-numbered speed gear ratio, another gear ratio to be put in gear to attain interlock is also an even-numbered speed gear ratio.

The relationship between the switching mechanism 16 and interlock will be described next. If a dog clutch is employed as the switching mechanism 16, interlock is not carelessly canceled as an advantage when such interlock is generated. FIG. 4A is a sectional view showing an example of the dog clutch. This is a sectional view taken along a plane including the input shaft. FIG. 4A shows an in-gear state.

A dog gear 101 with dog teeth 101 a is spline-connected to a gear member 100 that forms a transmission gear. A double-cone blocking ring 102 and a synchronizer spring 103 are provided between the dog gear 101 and a hub 105 fixed to the input shaft.

A sleeve 104 is provided to be reciprocally moved by the driving force of an actuator in an axial direction d1 of the input shaft. Spline teeth 104 a of the sleeve 104 can reciprocally move between dog teeth 102 a of the blocking ring 102 and between the dog teeth 101 a of the dog gear 101 while being guided between spline teeth 105 a of the hub 105. As shown in FIG. 4A, the spline teeth 104 a are placed across the spline teeth 105 a and the dog teeth 101 a, thereby attaining an in-gear state.

FIG. 4B is a sectional view taken along a plane in the circumferential direction passing through the spline teeth 104 a and the like. FIG. 4B shows the cross-sectional shape of the spline teeth 104 a and 105 a and the dog teeth 101 a and 102 a.

At the distal end of each spline tooth 104 a, the side surfaces are not parallel but are tilted to form a tapered shape. Each dog tooth 101 a has a tapered shape corresponding to the tapered shape so the side surfaces are not parallel but are tilted.

With this tapered shape, if a driving force in the direction of an arrow d2 acts on the gear member 100, the spline teeth 104 a and the dog teeth 101 a more deeply mesh with each other and hardly disengage. In the example of FIG. 3, if the vehicle stops on a climbing hill, the gear G3 that is the transmission gear for the third speed gear ratio and the structure of the switching unit 16 a corresponding to the sleeve 104 more deeply mesh with each other, and the gear G3 hardly disconnects.

On the other hand, when the vehicle starts, the interlock needs to be canceled. In the example of FIG. 4A, the sleeve 104 is retreated from the dog gear 101 by the driving force of an actuator (not shown) to attain an off-gear state. However, if an actuator whose output is relatively small is used, disengagement may be difficult because of the meshing between the spline teeth 104 a and the dog teeth 101 a.

However, if the driving force in the forward direction acts on the input shaft 11, the driving force gradually balances with the driving force of the backward movement caused by the weight of the vehicle on the climbing hill, and the meshing between the spline teeth 104 a and the dog teeth 101 a weakens to facilitate disengagement. That is, in the example of FIG. 3, vehicle start control is performed to, for example, set the clutch C1 in a half-engaged state while causing the switching unit 16 a to put the third speed gear ratio off gear. When the driving force of the driving source 2 is transmitted to the input shaft 11, and the driving force of the gravity on the climbing hill and the driving force from the driving source 2 balance, the resistance in the off-gear state by the meshing between the spline teeth 104 a and the dog teeth 101 a becomes almost 0, and the third speed gear ratio can reliably be put off gear. Hence, any special mechanism is not needed to cancel interlock.

<Control Examples>

Examples of control executed by the processing unit 61 regarding the above-described hill hold will be described with reference to FIGS. 5A to 5C. FIG. 5A shows an example of backward movement prevention control by interlock.

In step S1, it is determined whether a predetermined condition is met. Upon determining that a predetermined condition is met, the process advances to step S2. Upon determining that a predetermined condition is not met, the processing of one unit ends.

The predetermined condition includes at least the stop of the vehicle. The stop of the vehicle can be determined based on the detection result of the speed sensor 72. For example, if the detection result is less than a threshold, it can be determined that the vehicle has stopped.

The predetermined condition may include, for example, the stop of the vehicle on a climbing hill. If the stop of the vehicle on a climbing hill is included in the condition, the backward movement of the vehicle can be prevented without unnecessarily causing interlock. Whether the vehicle has stopped on a climbing hill can be determined based on the detection result of the slope sensor 73. For example, if a slope more than a predetermined slope is detected, it can be determined that the vehicle has stopped on a climbing hill. Conversely, an arrangement that does not use the stop of the vehicle on a climbing hill as a condition can also be employed. As described above, interlock can reliably be canceled by the start of the vehicle. Hence, interlock may always be generated at the time of the stop of the vehicle. If the stop of the vehicle on a climbing hill is not used as a condition, slope detection is unnecessary.

The predetermined condition may include, for example, selection of the D range. During selection of the D range, on a climbing hill, the necessity of hill hold is high at the time of release of the foot brake. In this case, the predetermined condition may further include detection of leg power reduction on the foot brake. This is because the possibility that the vehicle does not move back is high during braking using the foot brake, and at the start of release of the foot brake, the possibility of the backward movement rises. Leg power reduction on the foot brake can be determined based on the detection result of the brake sensor 75.

The predetermined condition may include non-detection of selection of the P range. At the time of lock by parking lock, the necessity of hill hold by interlock is low. The non-detection of selection of the P range may be determined at timing after the elapse of a predetermined time from the stop of the vehicle.

In step S2, the switching mechanism 16 is instructed to put one of the odd-numbered speed gear ratios other than the first speed gear ratio in gear, and the processing of one unit ends. The odd-numbered speed gear ratio to be put in gear at this time will be referred to as an IL odd-numbered speed gear ratio in the following explanation.

FIG. 5B shows an example of cancel control of canceling interlock after the backward movement prevention control. In step S11, it is determined whether the vehicle is at a stop, and the IL odd-numbered speed gear ratio is in gear. If YES in step S11, the process advances to step S12. Otherwise, the processing of one unit ends.

In step S12, it is determined whether a predetermined condition is met. Upon determining that a predetermined condition is met, the process advances to step S13. Upon determining that a predetermined condition is not met, the processing of one unit ends. Here, the predetermined condition is, for example, that the estimated value of the driving force required by the driver exceeds a threshold. The driving force required by the driver is the forward driving force of the vehicle required by the driver. The estimated value can be derived from, for example, the accelerator opening. The accelerator opening can be calculated based on the detection result of the accelerator position sensor 71. The clutch C1 and the driving source 2 can be driven and controlled in accordance with the estimated value of the driving force required by the driver. If the driving force input to the input shaft 11 increases, the IL gear ratio can readily be put off gear, as described with reference to FIG. 4B.

In step S13, the switching mechanism 16 is instructed to put the IL odd-numbered speed gear ratio off gear. The processing of one unit thus ends.

FIG. 5C shows another example of cancel control of canceling interlock after the backward movement prevention control. In the example of FIG. 5B, on condition that a predetermined condition is met in step S12, the IL gear ratio is put off gear in step S13. However, the IL gear ratio may be put off gear after execution of the backward movement prevention control before the vehicle start condition is met. The vehicle start condition may be, for example, detection of a driver's operation on the accelerator pedal by the accelerator position sensor 71.

In the example of FIG. 5C, after the IL gear ratio is put in gear to attain the interlock state, the off-gear operation of the IL gear ratio is performed immediately concerning the control.

First, in step S21, it is determined whether the vehicle is at a stop, and the IL odd-numbered speed gear ratio is in gear. If YES in step S21, the process advances to step S22. Otherwise, the processing of one unit ends. In step S22, the switching mechanism 16 is instructed to put the IL gear ratio off gear. The processing of one unit thus ends.

Step S22 means that the off-gear operation of the IL gear ratio is started concerning the control. In other words, it means that the operation of the actuator of the switching mechanism 16 that puts the IL gear ratio off gear is started. If the output of the actuator is small, the in-gear state of the IL gear ratio is maintained even after the start of the operation, as described with reference to FIG. 4B. Especially if the slope of the climbing hill is large, the in-gear state of the IL gear ratio is maintained even if off-gear control is performed. Conversely, if the slope of the climbing hill is small, the IL gear ratio may be put off gear by the off-gear control. In this case, however, the backward movement of the vehicle is considered not to occur.

Even if the in-gear state of the IL gear ratio is maintained, when the vehicle starts, at a timing at which the driving force of the backward movement by the gravity on the climbing hill and the driving force from the driving source 2 balance, the resistance in the off-gear state becomes almost 0, and the IL gear ratio is put off gear, as described with reference to FIG. 4B. By this control, the condition determination process of step S12 can be omitted.

<Prevention of Unintended Interlock>

The above-described generation of interlock is intentional. The interlock of this embodiment can be almost reliably canceled by inputting the driving force from the driving source 2 even if a strong jam occurs in the switching mechanism 16. However, generation of unintended interlock is not preferable. A case in which unintended interlock occurs and prevention of this will be described.

The automatic transmission 1 according to this embodiment may generate unintended interlock if the vehicle stops in an abrupt deceleration state. FIG. 6A is a timing chart showing the change of the gear ratio in normal deceleration. The in-gear state of the odd-numbered speed gear ratios and the vehicle speed are shown together.

FIG. 6A shows an example in which the driver operates the foot brake and stops the vehicle during selection of the eighth speed gear ratio. Note that the odd-numbered speed gear ratio in gear during traveling in the eighth speed gear ratio is the seventh speed gear ratio. However, it may be the ninth speed gear ratio.

In this embodiment, as already described, the gear ratio is normally switched one by one. In the example of FIG. 6A, the gear ratio is shifted down one by one in the order of eighth speed gear ratio→seventh speed gear ratio→ . . . second speed gear ratio→first speed gear ratio. After the first speed gear ratio is established, the vehicle speed is reduced to 0, and the vehicle stops.

FIG. 6B shows an example in which an excessive deceleration state (for example, at an acceleration of about −0.4 G) occurs in the vehicle. In the example of FIG. 6B as well, the gear ratio is shifted down one by one in the order of eighth speed gear ratio→seventh speed gear ratio→sixth speed gear ratio→. However, shift down to the first speed gear ratio cannot be done until the stop of the vehicle. During establishment of the third speed gear ratio, the vehicle speed is reduced to 0, and the vehicle stops. When the vehicle stops, a road surface reaction that makes the vehicle move back acts. In sudden braking, a relatively strong road surface reaction acts. Then, the interlock state described with reference to FIG. 3 occurs against a driver's intention. FIG. 6B shows the case of the third speed gear ratio. If the vehicle stops in a state in which the switching mechanism 16 is in gear for one of the third speed gear ratio, the fifth speed gear ratio, the seventh speed gear ratio, and the ninth speed gear ratio, and, by the road surface reaction, a driving force in the backward direction is input from the side of the wheels 5 to the automatic transmission 1, the interlock state occurs.

FIG. 7 shows switching control to prevent generation of such unintended interlock. After the driver operates the foot brake during selection of the eighth speed gear ratio, it is estimated whether shift down to the first speed gear ratio is possible until the stop of the vehicle. If it is estimated that the shift down is impossible, the switching mechanism 16 is set at the neutral position, and the transmission mechanism 10 selects the first speed gear ratio. This avoids a state in which the transmission mechanism 10 is in gear for one of the third speed gear ratio, the fifth speed gear ratio, the seventh speed gear ratio, and the ninth speed gear ratio when the vehicle stops.

For the transmission mechanism 20, the current gear ratio (the eighth speed gear ratio in the example of FIG. 7) is maintained. If the vehicle speed changes to a low speed (for example, 5 km/h), the gear ratio shifts down to the lowest ratio (here, the second speed gear ratio) of the transmission mechanism 20.

FIG. 8 shows an example of control executed by the processing unit 61 concerning the control example shown in FIG. 7. In step S31, it is determined whether a predetermined condition is met. Upon determining that the predetermined condition is met, the process advances to step S32. Upon determining that the predetermined condition is not met, the processing of one unit ends.

In the determination of step S31, it is determined whether the traveling state of the vehicle reaches a predetermined deceleration state. More specifically, it is determined whether the probability of generation of interlock described with reference to FIG. 6B is high. The predetermined condition includes at least a parameter associated with the traveling state of the vehicle, and can include, for example, the deceleration of the vehicle or the deceleration of the wheels 5. Depending on the state of the traveling road, the wheels 5 may slip. Hence, not the deceleration of the vehicle but the deceleration of the wheels 5 is preferably used as the criterion. The deceleration of the wheels 5 more than a threshold (high possibility of lock) is preferably used as the condition. The deceleration of the wheels 5 can be calculated based on the detection result of the wheel speed sensor 76.

The predetermined condition can also include the presence of an operation on the foot brake and an accelerator opening of 0. The former condition can be determined based on the detection result of the brake sensor 75. The latter condition can be determined based on the detection result of the accelerator position sensor 71.

The predetermined condition can also include that the transmission mechanism 10 has a gear ratio other than the first speed gear ratio. If the switching mechanism 16 of the transmission mechanism 10 is at a neutral position, no interlock occurs, and control to prevent it need not be performed.

The predetermined condition can also include that the traveling road is a climbing hill. This is because a stop on a climbing hill readily causes the above-described interlock. Whether the vehicle is traveling on a climbing hill can be determined based on the detection result of the slope sensor 73. For example, if a slope more than a predetermined slope is detected, it can be determined that the vehicle is traveling on a climbing hill. On the other hand, if the predetermined condition does not include that the traveling road is a climbing hill, slope detection is unnecessary, and the determination time can be shortened.

In step S32, the switching mechanism 16 is instructed to perform the off-gear operation for the transmission gear in the in-gear state, and the switching mechanism 16 is changed to the neutral position. In step S33, it is determined based on the detection results of the sensors whether the switching mechanism 16 completes the off-gear operation. If the off-gear operation is completed, the process advances to step S35. If the off-gear operation is not completed, the process advances to step S34.

In step S34, it is determined whether the target driving force exceeds a threshold. The target driving force is the forward driving force of the vehicle to be obtained by the control, and is set by, for example, the estimated value of the driving force required by the driver. In the process of step S34, it is determined whether to continue the deceleration state of the vehicle or shift to an acceleration state. If the target driving force exceeds the threshold, it is determined to shift to an acceleration state. The processing of one unit is ended, and the process returns to normal gear change control. If the target driving force does not exceed the threshold, the process returns to step S32 to continue the switching control.

In steps S35 to S38, processing associated with the transmission mechanism 20 is performed. In step S35, a gear ratio currently in the in-gear state is maintained. A traveling enable state can thus be at least maintained.

In step S36, it is determined whether the vehicle speed is less than a threshold. The vehicle speed can be determined based on the detection result of the vehicle speed sensor 72. The threshold can be set to, for example, 5 km/h, as described above. If the vehicle speed is less than the threshold, the process advances to step S37. If the vehicle speed is not less than the threshold, the process advances to step S38. In step S37, the gear ratio shifts down to the lowest ratio (in this embodiment, the second speed gear ratio) of the transmission mechanism 20 to prepare for the shift to the acceleration state, and the process returns to normal gear change control.

In step S38, it is determined whether the target driving force exceeds a threshold. This determination processing is the same as in step S34. If the target driving force exceeds the threshold, it is determined to shift to an acceleration state. The processing of one unit is ended, and the process returns to normal gear change control. If the target driving force does not exceed the threshold, the process returns to step S36 to continue the switching control.

SUMMARY OF EMBODIMENT

1. An automatic transmission (for example, 1) according to the embodiment comprises:

a first transmission mechanism (for example, 10) to which a driving force of a driving source (for example, 2) is input through a first clutch (for example, C1) and which is configured to switch driving force transmission paths to an output member (for example, 41) to establish a first set of gear ratios (for example, odd-numbered speed gear ratios);

a second transmission mechanism (for example, 20) to which the driving force of the driving source is input through a second clutch (for example, C2) and which is configured to switch driving force transmission paths to the output member to establish a second set of gear ratios (for example, even-numbered speed gear ratios); and

a control unit (for example, 60),

wherein a one-way clutch (for example, OC) is provided in a driving force transmission path that establishes a first certain gear ratio in the first set,

the first transmission mechanism comprises:

an input shaft (for example, 11) to which the driving force of the driving source is input through the first clutch;

a first transmission gear (for example, G1) provided on the input shaft and configured to establish the first certain gear ratio;

a plurality of second transmission gears (for example, G3, G5, G7, G9) provided on the input shaft and configured to establish remaining gear ratios in the first set; and

a switching mechanism (for example, 16) configured to perform engagement and disengagement between the input shaft and the plurality of second transmission gears,

a driving transmission direction of the one-way clutch is set such that a rotation inputted from a wheel side to the output member in a predetermined rotational direction is transmitted to the input shaft, the predetermined rotational direction corresponding to a backward movement of a vehicle, and

the control unit can execute switching control of causing the switching mechanism to perform the disengagement (for example, S32) between the input shaft and the plurality of second transmission gears on condition that a traveling state of the vehicle reaches a predetermined deceleration state (for example, S31).

According to the embodiment, generation of unintended interlock can be prevented.

2. In the automatic transmission (for example, 1) according to the embodiment,

the predetermined deceleration state is a state in which at least a deceleration of a wheel exceeds a threshold.

According to the embodiment, it is possible to prevent generation of interlock caused by a slip of wheels.

3. In the automatic transmission (for example, 1) according to the embodiment,

when the switching control is executed, the control unit maintains a selected gear ratio in the second set (for example, S35).

According to the embodiment, the traveling enable state can be at least maintained.

4. In the automatic transmission (for example, 1) according to the embodiment,

the control unit switches the gear ratio to a lowest ratio in the second set on condition that a vehicle speed becomes less than a threshold during the maintenance of the selected gear ratio (for example, S37).

According to the embodiment, the traveling state can quickly be shifted to an acceleration state.

5. In the automatic transmission (for example, 1) according to the embodiment,

the switching mechanism comprises:

a dog clutch; and

an actuator configured to operate the dog clutch.

According to the embodiment, it is possible to prevent interlock caused by a jam in the mechanism.

6. In the automatic transmission (for example, 1) according to the embodiment,

the gear ratios of the first set are odd-numbered speed gear ratios, and

the gear ratios of the second set are even-numbered speed gear ratios.

According to the embodiment, the interlock generated in an odd-numbered speed gear ratio can be prevented. Additionally, traveling performance can be ensured on the even-numbered speed gear ratio side.

7. In the automatic transmission (for example, 1) according to the embodiment,

if the traveling state of the vehicle is a deceleration state different from the predetermined deceleration state, the control unit shifts down the gear ratio one by one (for example, FIG. 6A).

According to the embodiment, it is possible to perform shift down while making the load on the mechanism smaller.

8. In the automatic transmission (for example, 1) according to the embodiment,

the first gear ratio is a 1^(st)-speed gear ratio.

According to the embodiment, the cost can be reduced by employing the one-way clutch.

9. The automatic transmission (for example, 1) according to the embodiment further comprises a countershaft provided in parallel to the input shaft,

the output member is provided on the countershaft, and

a gear configured to mesh with the first transmission gear is provided on the countershaft through the one-way clutch.

According to the embodiment, the interlock generated by a torque circulation between the input shaft and the countershaft can be prevented.

10. A control method, according to the embodiment, of an automatic transmission (for example, 1) which comprises:

a first transmission mechanism to which a driving force of a driving source is input through a first clutch and which is configured to switch driving force transmission paths to an output member to establish a first set of gear ratios; and

a second transmission mechanism to which the driving force of the driving source is input through a second clutch and which is configured to switch driving force transmission paths to the output member to establish a second set of gear ratios,

wherein a one-way clutch is provided in a driving force transmission path that establishes a first certain gear ratio in the first set,

the first transmission mechanism comprises:

an input shaft to which the driving force of the driving source is input through the first clutch;

a first transmission gear provided on the input shaft and configured to establish the first certain gear ratio;

a plurality of second transmission gears provided on the input shaft and configured to establish remaining gear ratios in the first set; and

a switching mechanism configured to perform engagement and disengagement between the input shaft and the plurality of second transmission gears, and

a driving transmission direction of the one-way clutch is set such that a rotation inputted from a wheel side to the output member in a predetermined rotational direction is transmitted to the input shaft, the predetermined rotational direction corresponding to a backward movement of a vehicle,

the control method comprises:

determining whether a traveling state of the vehicle reaches a predetermined deceleration state (for example, S31); and

upon determining that the traveling state reaches the predetermined deceleration state, executing switching control of causing the switching mechanism to perform the disengagement between the input shaft and the plurality of second transmission gears (for example, S32).

According to the embodiment, generation of unintended interlock can be prevented.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefits of Japanese Patent Application No. 2016-037932, filed Feb. 29, 2016, which is hereby incorporated by reference herein in its entirety. 

What is claimed is:
 1. An automatic transmission comprising: a first transmission mechanism to which a driving force of a driving source is input through a first clutch and which is configured to switch driving force transmission paths to an output member to establish a first set of gear ratios; a second transmission mechanism to which the driving force of the driving source is input through a second clutch and which is configured to switch driving force transmission paths to the output member to establish a second set of gear ratios; and a control unit, wherein a one-way clutch is provided in a driving force transmission path that establishes a first certain gear in the first set, the first transmission mechanism comprises: an input shaft to which the driving force of the driving source is input through the first clutch; a first transmission gear provided on the input shaft and configured to establish the first certain gear ratio; a plurality of second transmission gears provided on the input shaft and configured to establish remaining gear ratios in the first set; and a switching mechanism configured to perform engagement and disengagement between the input shaft and the plurality of second transmission gears, a driving transmission direction of the one-way clutch is set such that a rotation inputted from a wheel side to the output member in a predetermined rotational direction is transmitted to the input shaft, the predetermined rotational direction corresponding to a backward movement of a vehicle, and the control unit can execute switching control of causing the switching mechanism to perform the disengagement between the input shaft and the plurality of second transmission gears on condition that a traveling state of the vehicle reaches a predetermined deceleration state.
 2. The transmission according to claim 1, wherein the predetermined deceleration state is a state in which at least a deceleration of a wheel exceeds a threshold.
 3. The transmission according to claim 1, wherein when the switching control is executed, the control unit maintains a selected gear ratio in the second set.
 4. The transmission according to claim 3, wherein the control unit switches the gear ratio to a lowest ratio in the second set on condition that a vehicle speed becomes less than a threshold during the maintenance of the selected gear ratio.
 5. The transmission according to claim 1, wherein the switching mechanism comprises: a dog clutch; and an actuator configured to operate the dog clutch.
 6. The transmission according to claim 1, wherein the gear ratios in the first set are odd-numbered speed gear ratios, and the gear ratios of the second set are even-numbered speed gear ratios.
 7. The transmission according to claim 6, wherein if the traveling state of the vehicle is a deceleration state different from the predetermined deceleration state, the control unit shifts down the gear ratio one by one.
 8. The transmission according to claim 6, wherein the first certain gear ratio is a 1^(st)-speed gear ratio.
 9. The transmission according to claim 1, further comprising a countershaft provided in parallel to the input shaft, wherein the output member is provided on the countershaft, and a gear configured to mesh with the first transmission gear is provided on the countershaft through the one-way clutch.
 10. A control method of an automatic transmission which comprises: a first transmission mechanism to which a driving force of a driving source is input through a first clutch and which is configured to switch driving force transmission paths to an output member to establish a first set of gear ratios; and a second transmission mechanism to which the driving force of the driving source is input through a second clutch and which is configured to switch driving force transmission paths to the output member to establish a second set of gear ratios, wherein a one-way clutch is provided in a driving force transmission path that establishes a first certain gear ratio in the first set, the first transmission mechanism comprises: an input shaft to which the driving force of the driving source is input through the first clutch; a first transmission gear provided on the input shaft and configured to establish the first certain gear ratio; a plurality of second transmission gears provided on the input shaft and configured to establish remaining gear ratios in the first set; and a switching mechanism configured to perform engagement and disengagement between the input shaft and the plurality of second transmission gears, and a driving transmission direction of the one-way clutch is set such that a rotation inputted from a wheel side to the output member in a predetermined rotational direction is transmitted to the input shaft, the predetermined rotational direction corresponding to a backward movement of a vehicle, the control method comprising: determining whether a traveling state of the vehicle reaches a predetermined deceleration state; and upon determining that the traveling state reaches the predetermined deceleration state, executing switching control of causing the switching mechanism to perform the disengagement between the input shaft and the plurality of second transmission gears. 